Interpretive Summary: Jasmonic acid (JA) reduces rot due to several sugarbeet storage pathogens. However, the mechanisms by which JA protects postharvest sugarbeet roots from disease are unknown. In other plants, increases in enzymes and metabolites that protect plants from oxidative stress or pathogens are thought to be involved in JA-induced disease resistance. These enzymes and metabolites, however, were largely unaffected by JA treatment in stored sugarbeet roots indicating that they are not directly involved in induced disease resistance in this crop. Enzymes that protect plants from oxidative stress or pathogens, however, were affected by storage duration. Enzyme activities increased after prolonged storage when disease resistance declines, providing further evidence that these enzymes are unlikely to be involved in sugarbeet root disease resistance.

Technical Abstract:
Jasmonic acid (JA) treatment significantly reduces rot due to several sugarbeet (Beta vulgaris L.) storage pathogens. However, the mechanisms by which JA protects postharvest sugarbeet roots from disease are unknown. In other plant species and organs, alterations in antioxidant defense mechanisms and elevations in common pathogen-related defense enzymes have been implicated in jasmonate-induced disease resistance. To investigate whether these mechanisms are involved in JA-induced disease resistance in stored sugarbeet roots, the activities of several reactive oxygen species (ROS)-scavenging and pathogen-related defense enzymes and the total concentration of antioxidant compounds were determined in harvested sugarbeet roots in the 60 d following treatment with JA. ROS-scavenging and pathogen-related defense enzymes and the concentration of antioxidant compounds were largely unaffected by JA as JA-treated roots exhibited small declines in superoxide dismutase (SOD) and chitinase activities, and were generally unaltered in ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), ß-1,3-glucanase (ß-Gluc), polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL) activities or antioxidant compounds concentration. The lack of increase in enzyme activities or metabolites related to defense against oxidative stress or pathogens suggests that JA-induced disease resistance in postharvest sugarbeet roots does not arise from a direct increase in any of the ROS-scavenging and defense-related enzymes examined, or the concentration of total antioxidant compounds. However, ROS-scavenging enzymes and pathogen-related defense enzymes were affected by storage duration with POD, SOD, ß-Gluc, chitinase, and PPO activities elevated and APX and CAT activities reduced in roots stored for 10 d or more. Storage-related changes in activities of ROS-scavenging enzymes and defense-related enzymes provide further evidence that these enzymes are uninvolved in sugarbeet root disease resistance since many of these enzymes increased in activity after prolonged storage when disease resistance generally declines.